1. Field of the Invention
The invention relates to electrical control systems and, more particularly, to an electrical control system and driver which repeatedly applies power to and removes power from an electrical load to reduce the energy consumed by the load or to control its output.
2. Description of the Prior Art
As will be readily appreciated by those skilled in the art of electrical load control systems, the present invention can be utilized with a wide variety of types of electrical load apparatus. Accordingly, while the discussion of the invention and the prior art is primarily directed toward the control of fluorescent lights and fluorescent lighting systems, such a discussion is not to be construed as a limitation on the application of the present invention.
A gas discharge type lamp and its associated ballast are among the most difficult electrical loads to regulate and control. The term gas discharge lamp includes a fluorescent lamp with or without a separate heater, a high intensity discharge lamp and any lamp which typically exhibits a negative resistance characteristic. Such a lamp requires a ballast circuit to provide a stable operating condition when it is used with a standard AC power source. The ballast also provides additional striking voltage to start the lamp and, in some cases, to provide power for internal lamp cathode heaters. A good discussion of gas discharge lamps and the problems associated with controlling them can be found in U.S. Pat. No. 4,352,045 at column 1, line 22, through column 2, line 9.
It has been a long-standing objective to provide a control system for dimming gas discharge lamp assemblies to reduce energy consumption and the associated cost of operating the lamps. Generally, dimming is appropriate whenever the standard available light output of the lamps is not required due to ambient light conditions and desired light level.
Conventional approaches to gas discharge lamp control, such as those disclosed in U.S. Pat. Nos. 4,350,935 and 4,352,045, have produced, for example, circuits which disconnect the energizing voltage from the load during a portion of each cycle of the AC supply voltage and subsequently reapply energizing voltage to the load to effectively remove a portion of the energizing voltage from the load during each half cycle of supply voltage. The rapid potential change due to removal of supply voltage from the load causes a countereffect of control which is called back electromotive force, or EMF. The time at which all such conventional circuits reapply energizing voltage to the load during a half cycle of the supply voltage does not depend on the relative electrical potential between the load, due to back EMF, and the supply voltage. Accordingly, the conventional circuits nearly always reapply energizing voltage to the load when an electric potential exists between the supply voltage and the load that creates an extremely large and damaging voltage transient at the time the energizing voltage is reapplied to the load. The voltage transient can cause immediate destruction or shortened life of the control components and can create annoying audible noise.
Another problem is caused by the very fast switching times employed by many conventional circuits, relative to the 50 to 60 Hz voltage being supplied to the ballasts, which can cause shortened ballast life. Also, removing and applying energizing voltage from the load prior to the 90.degree. point of each half cycle of the supply voltage causes high crest factors and very fast voltage changes which stress the ballast and lamp. Increased noise results from significant swings in voltage during the half cycle, which produce multiple harmonics within the audible range. Switching prior to the 90.degree. point in each half cycle of the supply voltage causes the further problem that voltage of an amplitude sufficient to ignite the lamps may not be applied to the load, which will cause some lamps to be extinguished completely.
Accordingly, there is a need for a driver for the control system for a load that does not produce transients that are harmful to the load or the control components. Further, there is a need for a driver and control system for a load having an inductive component that does not produce any large voltage transients when the energizing voltage is applied to or removed from the load.